Friday, November 2, 2007

Pixels and screen capture

Whenever an instructor asks an imaging class about the definition of a pixel, someone will always shout out: "a picture element." That's fine ... but what does that mean? Does it represent physical matter? Does it occupy a specific point in space? Does it have mass or volume? With all of this in mind, just what is a pixel?

First of all, a pixel doesn't have a particular size. It is an "abstract representation of a specific coordinate," like a point on a map. That is to say, a pixel is a point on a grid - or to combine the two definitions into one ... a pixel is a picture element on a grid. We've said nothing thus far about a possible size of the point. 

Q: So how do we get images from pixels? 
A: Pixels are used to create a pattern.

Q: What about size? You know pixels per inch, samples per inch, and dots per inch.
A: It depends ... Samples per inch is the measurement of the scanner's world. Dots per inch refers to the amount of ink used on a page in the printing world. For screen capture, we are concerned with pixels per inch.

Q: So what happens when I use a screen capture program? What will be the resolution of the images that my program captures? Everyone assumes that screens display at 72 PPI. Is this the case?
A: Again, it depends ... Take a ordinary tape measure and find the horizontal measurement of your monitor. Use the formula, horizontal resolution/monitor width = PPI. Here's how it looks:

I have a 17" wide screen laptop set at 1400x900. I find first that my 17" monitor isn't exactly 17", its closer to 16.7. I plug the numbers into the formula and find that my screen capture image resolution should end up around 85 PPI. Here are some common sizes and resolutions from around my work area:

21" LCD @ 1600x1200 = 95 PPI
21" LCD @ 1280x1024 = 76 PPI

Q: Why the change?
A: The horizontal resolution decreased, so the PPI decreased.

21" CRT @ 1920x1440 = 120 PPI
21" CRT @ 1600x1200 = 100 PPI
21" CRT @ 1280x1024 = 82 PPI

Q: Why does a CRT give you more pixels at the same setting vs. a CRT?
A: That's a discussion for another time. Suffice it to say that as time goes on, you won't have to worry about CRT's much, if you can find one.

So, when you are setting up your system for screen capture, bear this all in mind. Try to find a GPU and monitor combination that will support higher horizontal resolutions. Work with your software's manufacturer to find the highest resolution settings supported. Can't get new gear? You may have some extra PPI that you can squeeze out of your existing equipment. Make some adjustments and experiment.

Enjoy the weekend ...

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